Tube hydroforming (THF) is relatively a new process for manufacturing structural components in various industries, such as aerospace, automotive and marine. Compared to the traditional manufacturing processes, such as stamping and welding, this technique presents many advantages, such as lower weight to rigidity ratio in the component, better stress distribution in the resulting part, and less effort to form joints or contours, including complex shapes. Moreover, THF can provide sharper corners and more easily produces a variety of shapes that are difficult to produce with the forming and welding of several parts.
A schematic illustration of a hydroforming press is shown in FIG. 1. In this process, tubular blank 10 is placed inside a die 12 and the die is closed. While the blank 10 shown is straight, with circular cross-section, it is known to use a pre-bent/preformed blank with variety of cross sections, and while the die 12 shown has symmetric walls, it is known that a wide variety of die shapes are possible. Then, the ends of the tube are sealed by end plungers 15 which have channels 16 therein for injection and venting of a pressurized fluid (which may be water, as the name suggests, but may be other liquids or gasses). While the end plungers 15 seal against the interior volume of the blank 10, the pressurized fluid is applied, and the blank 10 plastically deforms until it takes the shape of the die cavity. The pressurized fluid is vented, the die is opened, and the formed piece is removed.
One of the greatest limitations on THF is the formability of the material. To conform to a die wall that is substantially far from the unstressed blank wall, the blank material needs to exhibit considerable ductile strain. After the material's elongation is spent, the tube cannot safely deform any more without rupturing. Even before rupture, selective thinning in the areas where the blank is deformed, generally leads to a weakening of the material and may not be desirable. To control thickness at critical locations in hydroformed parts, either the blank material needs to be changed (i.e. replaced with a more deformable material) or the thickness of the material needs to be increased. In many applications, neither of these options is preferable, since both can result in changes in the mechanical properties, cost, weight and service behavior of the final product. Typically it would be preferred to increase the formability of the material during the hydroforming process by feeding more material to the expanding/deforming zone. This can be realized by applying more force at the ends of the tube than is required to maintain a seal between the tube and plungers.
In this way, the tube ends migrate towards a centre of the die cavity, thereby feeding more material to the deforming zone. This is called end feeding and is well known and addressed in the open literature. Thus, it is known to applying a compressive force on the blank 10 via end plungers 15 during the pressurization and deformation. The compressive loading adds some complexity to the requirements for the end plungers 15 and die 12, but can substantially improve a range of products and properties of the products that can be derived from a tube hydroforming apparatus.
Unfortunately, there are several limitations on how much end feeding a system can provide. Not the least of the problems is that of friction in nip areas 17 of the system, i.e., where the end plungers 15 seal against the blank 10, and the blank 10 is supported by the die 12 to prevent expansion of the blank 10 near its ends. Fluid pressure at the end feeding region of the tube is equal to the pressure applied inside the tube. Here a substantial advantage of the hydroforming process—the fact that uniform pressure is applied evenly across the internal surface of the blank 10—works against the desire for substantial end feeding, in that the friction resists end feeding, requiring greater force, this friction wears out the cavity in the nip areas 17, as well as the end plungers 15.
Furthermore, in typical conventional end feeding, in most cases, the material begins to fold or to get thicker (crumpling or thickening) at the end feeding region due to the friction leading to further artifacts that are typically not desired in the end products (which may contribute to wasted material that needs to be removed) further reduces the efficiency of the end feeding, in that less fed material is driven into the cavity centre. This further requires more force to be applied to the tube ends for a given desired amount of end feeding being delivered further into the die. Again this further force exacerbates the friction and the folding.
End feeding does not have to be limited to THF. Any effectively closed chamber with one or more openings that can be sealed to form a closed cavity can be formed with an analogous hydroforming technique, and end feeding can be applied whenever there is sufficient support for the end feeding to encounter. The availability and low cost production of tubular blanks, and the fact that tubes provide opposing ends that allow for the countering of two end feeding forces make THF a primary application for the present invention.
What is therefore needed is a technique for end feeding that reduces the friction in the nip area, thereby allowing for more end feeding and/or less wear on the parts in the nip area.